Philippine President Rodrigo Duterte has suffered his first major drop in popularity, an independent pollster found, after the deaths of two Manila teenagers in his controversial anti-crime crackdown triggered mass protests.

The decline in support shown by the Social Weather Stations survey -- down to 67 percent from 78 percent in June who said they were "satisfied" with Duterte's performance as president -- came amid growing public outrage over drug war killings in the Philippines.

Duterte assumed office in June last year vowing to eradicate illegal drugs in society by killing up to 100,000 traffickers and addicts.

In the 15 months since he came to power police have reported killing more 3,850 people in anti-drug operations. Thousands of other people have been murdered in unexplained circumstances that rights groups partly attribute to vigilante death squads.

Although Duterte's drug war remains largely popular among a citizenry fed up with high crime, growing unease with the killings resulted in August and September in large street protests over the deaths of two Manila teenagers.

The Manila-based pollster, which has been regularly running the quarterly surveys since 1986, did not explain the drop shown in Sunday's figures. But analysts told AFP it was likely linked to growing public concern over the drug war killings.

"It may likely reflect growing concerns over the direction of (the) war on drugs, particularly the death of minors and indiscipline within ranks of the law enforcement agencies," political analyst Richard Heydarian said.

State prosecutors are studying the possible filing of murder charges against four anti-drugs officers over the August 16 shooting death of Manila student Kian delos Santos, 17.

Several other officers are under investigation after they killed alleged robbery suspect Carl Angelo Arnaiz, 19, the following night. His neighbour Reynaldo de Guzman, 14, was later found murdered by unknown suspects in the northern Philippines.

"It's the first significant slide since the president was inaugurated," said Victor Andres Manhit, chief executive of Manila-based political consultancy Stratbase ADR Institute, of the survey.

Duterte's aides largely shrugged off the new numbers.

"The love is still there," Duterte spokesman Ernesto Abella told reporters, adding the poll numbers were consistent with the pattern of previous Filipino presidents who saw dips in their popularity after their first year in office.

Heydarian, who has recently published a book titled "Rise of Duterte", said the lower poll numbers still left the president with "relatively high approval ratings and enough momentum" to pursue his political agenda.

Social Weather Stations said Duterte's declining popularity among Filipino poor largely accounted for his poll drop.

Manhit said: "They are his main constituents, but they are also the main victims of the drug war."

A representative nationwide sample of 1,500 adults were asked in person between September 23-27: "Please tell me how satisfied or dissatisfied you are in the performance of Rodrigo Duterte as president of the Philippines."

Social Weather Stations said it conducted the poll for itself and was not commissioned by any other party.

The number of dissatisfied over Duterte also rose by seven percentage points from June to 19 percent, with the undecided up by four points to 14 percent, it added.